Eco challenges are the same levels that you already played through, but you have a lower used-bamboo requirement. To finish the levels using less bamboo takes a little more thought, but isn't too hard. Here's how I did it.
I've taken pictures of all of my winning level configurations, but that may not directly equate to beating the levels for you. I found that the way things fall, the monkey's effect on the sticks, and the final outcome of the level are non-deterministic. I would occasionally beat a level without changing anything - just restarting the action seemed to make it work. So, take the pictures as a starting point,a nd adjust them as needed to get to the winning combo.
For the pictures, they were taken after I beat the Eco challenge for each level, so they will show you how I got this achievement. Also, some levels needed two pictures, so you will see some a/b pictures. a is generally the top of the level, and b is the bottom.
http://tikitowers.4shared.com/Also in that link is a complete list of islands, levels, and eco targets for left over bamboo.
Here is the list of levels for the first island with their target eco challenge savings numbers in parenthesis:
The Blue Lagoon
1-1 - How to Build a Tower (6)
1-2 - Bridge Building Basics (4)
1-3 - Advanced Bridge Building (8)
2-1 - Mixing It Up (8)
2-2 - Up and Down (9)
2-3 - Leap of Faith (4)
3-1 - Hanging Out (12)
3-2 - Do You Swing (10)
3-3 - Balance of Tower (2)